The invention particularly relates to such a syringe which is constructed relatively inexpensively and is therefore intended to be discarded when the product with which it has been filled has been exhausted.
Conventional disposable syringes contain only a single dose of the drug concerned, but in certain cases multi-dose syringes are advantageous, especially where a patient has to administer doses of the drug himself over a period of time, possibly at a frequency of more than one dose per day. In such a case the total daily dose, or even the total dose for more than one day, could be contained in a single disposable syringe to be administered in stages as required.
One multi-dose syringe is described in W088/07874. It includes a housing for a phial of liquid to be injected, having adjustment means for the dose incorporating a stop to limit the stroke of a plunger for forcing the injectable liquid out of the phial through a needle. The position of the stop is variable by rotation of the adjustment means. Ratchets are provided to ensure that the adjustment means can be rotated only in one direction and that the plunger can be displaced only in one direction. A protective cap is provided for the needle and this can be removed and fitted to the opposite end of the syringe so that an indicator line on the cap can be used as a guide to assist in a zeroing dose measuring scale associated with the adjustment means. Injection is accomplished by pressing on the cap when fitted to the end of the syringe opposite the needle.
This syringe suffers from several disadvantages. Firstly, the protective cap must be removed from the needle before the dose can be set. The needle is susceptible to damage at this time and it would be preferable to be able to set the dose with the cap still fitted over the needle and thus protecting it. Also, it is difficult to carry out injection with the cap still fitted over the end opposite needle but any attempt to remove the cap prior to injection is likely to cause undesired premature discharge of liquid through the needle. Furthermore, the ratchet mechanisms employed in W088/07874 require great precision in manufacture and in practice are likely to be the source of imprecise dosing.